Radio-frequency surgical appliances have a neutral electrode which makes contact with the patient to be treated, and have a so-called active electrode, which is used at the operation point. The radio-frequency current used for surgery flows from a generator, via the active electrode and through the patient's body and the neutral electrode back to the generator. As far as possible, the entire arrangement in modern RF surgical appliances is designed not to be grounded.
Nevertheless, in such an arrangement, a certain amount of leakage current always flows, both from the active electrode and from the neutral electrode, with respect to the ground potential. This is inevitably the case owing to the capacitances to ground potential that are always present. On the active electrode side, such capacitances which are, for example, in the form of a so-called ground capacitance from the output transformer and from circuit parts for monitoring and control in the output circuit as well as internal wires on the active electrode side, are on the order of magnitude of up to 100 pF or more. The connecting cable to the active electrode handpiece also has a ground capacitance on the order of magnitude of several tens of picofarads. On the neutral electrode side, a ground capacitance is formed from the output transformer and from circuit parts for monitoring as well as for internal wires on the neutral electrode side, and is on the order of magnitude of several tens of picofarads. Furthermore, the connecting cable for the neutral electrode in this case has a ground capacitance of several tens of picofarads, and the neutral electrode as well as the patient connected to it have a ground capacitance which may be up to several nanofarads.
The output voltage U.sub.HF from the said generator produces a ground current via the capacitances on the active electrode side, and this current flows back to the voltage source via the capacitances arranged on the neutral electrode side. Thus, leakage currents are also produced on the neutral electrode side itself, and these leakage currents represent a problem since the neutral electrode is connected directly to the patient, who is at a potential close to ground potential, as a result of which parasitic currents are possible whose profile cannot be defined exactly. Although, as a rule, the patients are supported such that they are insulated from ground potential, owing to his or her capacitance, the patient is very close to ground potential for a radio-frequency current. The insulated support merely reduces the risk of so-called current bottlenecks being formed at ground contact points, at which considerable heating can occur.
The leakage currents which are present owing to the ground capacitance of the patient may cause undesirable effects, and should thus be prevented if possible.
To this end, DE 27 40 751 has disclosed a safety circuit which produces an alarm if this leakage current exceeds a threshold value. This threshold value is itself controlled by a circuit so that fluctuations in the generator output power are compensated. However, this apparatus only allows the electrical surgical procedure to be terminated or interrupted as soon as the threshold value is exceeded and the undesirable leakage currents are too high.
Furthermore, DE 94 90 451 has disclosed an apparatus and a method by means of which the magnitude of the leakage currents is measured, and the output power of the radio-frequency generator is controlled. This apparatus thus represents active current limiting, although the current limiting may be restricted to the output power of the generator in such a way that the electrical surgical treatment is once again adversely affected.
In the past, so-called Type B appliances existed, in which the neutral electrode was solidly grounded. In this case, there were no leakage currents from the neutral electrode to ground, since they had already been shorted to ground in the appliance. However, this type of circuit results in a safety risk. If a defect occurs in another appliance connected to the patient, for example an EKG monitor, mains power could reach the patient, in the worst case, as a result of an insulation fault. This voltage with respect to ground would then lead to a lethally high current via the neutral electrode owing to the fact that the patient was directly grounded. This type of circuit is, (comma) therefore, (comma) no longer permissible and, instead of this, a capacitor is inserted in the ground line from the neutral electrode, which capacitor forms a negligible, low resistance for the RF current but limits the mains-frequency fault current to a safe level of max. 100 .mu.A. Such appliances are called Type BF (body floating).
Such low-frequency currents are much too high for operations in the thorax region, so that appliances have been created in which the neutral electrode no longer has any direct ground reference. Such appliances are called Type CF (Cardial Floating) and are prior art in the OP field. In fact, there is now a problem here since the neutral electrode can assume an RF potential with respect to ground, as a result of which it is possible for leakage currents to flow on the neutral electrode side once again.
Undesirable leakage currents may occur not only when the described electrical surgical appliances are used but also in general with all electrical medical appliances.